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Re: [Suggestion] Nho'm Telecom
Chao cac anh chi tren VNSA va ca nha Telecom,
Rat vui mung duoc lam quen voi cac anh chi nao co dinh dang den Telecom.
Toi xin duoc ra nhap nhom Telecom do anh Le Hai Nam khoi xuong. Vai dong
ve ban than: Nguyen Hoang Ha, Telecom program, AIT (May term 1996), truoc
khi di hoc da cong tac tai Vietnam Telecom International (12 lang Trung -
Hanoi) duoc 8 thang. Research interests: Digital communications,
Coded-Modulation Techniques for radio fading channels, Error-Control
Coding in digitalcommunications systems.
> To^i dde^` nghi. cu+? a. Le^ Ha?i Nam (Japan) la`m ho^.i tru+o+?ng ho^.i
> Telecom (vnsa). Ly' do: anh Nam co`n ra^'t tre?, dda^`y na(ng lu+.c, dde.p trai,
> chu+a vo+..
Anh Nam la senior cua toi tai AIT (va hinh nhu ca anh Hoang o Ao nua, Alo
co anh Hoang o do khong ?). Hoan toan dong y voi Tuan ve cac diem manh cua
anh Nam nhung xin anh Tuan kiem chung lai khoan 'chu+a vo+.' de khoi lam
nhieu chi em mung hut...
Thay cac anh ban nhieu ve Mobile phone xin co mot bai ngan gioi
thieu qua ve CDMA va cac chuan mobile hien co.
(PHS khong thuoc ho CELLULAR mobile phones nen khong duoc de cap)
=============================================================================
What is CDMA?
One of the most important concepts to any cellular telephone system is
that of "multiple access", meaning that multiple, simultaneous users can
be supported. In other words, a large
number of users share a common pool of radio channels and any user can
gain access to any channel (each user is not always assigned to the same
channel). A channel can be thought of
as merely a portion of the limited radio resource which is temporarily
allocated for a specific purpose, such as someone's phone call. A multiple
access method is a definition of how the
radio spectrum is divided into channels and how channels are allocated to
the many users of the system.
Current Cellular Standards
Different types of cellular systems employ various methods of multiple
access. The traditional analog cellular systems, such as those based on
the Advanced Mobile Phone Service
(AMPS) and Total Access Communications System (TACS) standards, use
Frequency Division Multiple Access (FDMA). FDMA channels are defined by a
range of radio frequencies,
usually expressed in a number of kilohertz (kHz), out of the radio
spectrum.
For example, AMPS systems use 30 kHz "slices" of spectrum for each
channel. Narrowband AMPS (NAMPS) requires only 10 kHz per channel. TACS
channels are 25 kHz wide. With
FDMA, only one subscriber at a time is assigned to a channel. No other
conversations can access this channel until the subscriber's call is
finished, or until that original call is handed off
to a different channel by the system.
A common multiple access method employed in new digital cellular systems
is the Time Division Multiple Access (TDMA). TDMA digital standards
include North American Digital
Cellular (know by its standard number IS-54), Global System for Mobile
Communications (GSM), and Personal Digital Cellular (PDC).
TDMA systems commonly start with a slice of spectrum, referred to as one
"carrier". Each carrier is then divided into time slots. Only one
subscriber at a time is assigned to each time
slot, or channel. No other conversations can access this channel until the
subscriber's call is finished, or until that original call is handed off
to a different channel by the system.
For example, IS-54 systems, designed to coexist with AMPS systems, divide
30 kHz of spectrum into three channels. PDC divides 25 kHz slices of
spectrum into three channels. GSM
systems create 8 time-division channels in 200 kHz wide carriers.
The CDMA Cellular Standard
With CDMA, unique digital codes, rather than separate RF frequencies or
channels, are used to differentiate subscribers. The codes are shared by
both the mobile station (cellular phone)
and the base station, and are called "pseudo-Random Code Sequences." All
users share the same range of radio spectrum.
For cellular telephony, CDMA is a digital multiple access technique
specified by the Telecommunications Industry Association (TIA) as "IS-95."
In March 1992, the TIA established the TR-45.5 subcommittee with the
charter of developing a spread-spectrum digital cellular standard. In July
of 1993, the TIA gave its approval of the
CDMA IS-95 standard.
IS-95 systems divide the radio spectrum into carriers which are 1,250 kHz
(1.25 MHz) wide. One of the unique aspects of CDMA is that while there are
certainly limits to the number of
phone calls that can be handled by a carrier, this is not a fixed number.
Rather, the capacity of the system will be dependent on a number of
different factors. This will be discussed in
later sections.
CDMA Technology
Though CDMA's application in cellular telephony is relatively new, it is
not a new technology. CDMA has been used in many military applications,
such as anti-jamming (because of the
spread signal, it is difficult to jam or interfere with a CDMA signal),
ranging (measuring the distance of the transmission to know when it will
be received), and secure communications
(the spread spectrum signal is very hard to detect).
Spread Spectrum
CDMA is a "spread spectrum" technology, which means that it spreads the
information contained in a particular signal of interest over a much
greater bandwidth than the original signal.
A CDMA call starts with a standard rate of 9600 bits per second (9.6
kilobits per second). This is then spread to a transmitted rate of about
1.23 Megabits per second. Spreading means
that digital codes are applied to the data bits associated with users in a
cell. These data bits are transmitted along with the signals of all the
other users in that cell. When the signal is
received, the codes are removed from the desired signal, separating the
users and returning the call to a rate of 9600 bps.
Traditional uses of spread spectrum are in military operations. Because of
the wide bandwidth of a spread spectrum signal, it is very difficult to
jam, difficult to interfere with, and difficult
to identify. This is in contrast to technologies using a narrower
bandwidth of frequencies. Since a wideband spread spectrum signal is very
hard to detect, it appears as nothing more than
a slight rise in the "noise floor" or interference level. With other
technologies, the power of the signal is concentrated in a narrower band,
which makes it easier to detect.
Increased privacy is inherent in CDMA technology. CDMA phone calls will be
secure from the casual eavesdropper since, unlike an analog conversation,
a simple radio receiver will not
be able to pick individual digital conversations out of the overall RF
radiation in a frequency band.
Synchronization
In the final stages of the encoding of the radio link from the base
station to the mobile, CDMA adds a special "pseudo-random code" to the
signal that repeats itself after a finite amount
of time. Base stations in the system distinguish themselves from each
other by transmitting different portions of the code at a given time. In
other words, the base stations transmit time
offset versions of the same pseudo-random code. In order to assure that
the time offsets used remain unique from each other, CDMA stations must
remain synchronized to a common
time reference.
The Global Positioning System (GPS) provides this precise common time
reference. GPS is a satellite based, radio navigation system capable of
providing a practical and affordable
means of determining continuous position, velocity, and time to an
unlimited number of users.
"The Balancing Act"
CDMA cell coverage is dependent upon the way the system is designed. In
fact, three primary system characteristics-Coverage, Quality, and
Capacity-must be balanced off of each
other to arrive at the desired level of system performance.
In a CDMA system these three characteristics are tightly inter-related.
Even higher capacity might be achieved through some degree of degradation
in coverage and/or quality. Since
these parameters are all intertwined, operators cannot have the best of
all worlds: three times wider coverage, 40 times capacity, and "CD"
quality sound. For example, the 13 kbps
vocoder provides better sound quality, but reduces system capacity as
compared to an 8 kbps vocoder.
CDMA Benefits
When implemented in a cellular telephone system, CDMA technology offers
numerous benefits to the cellular operators and their subscribers. The
following is an overview of the benefits
of CDMA.
1.Capacity increases of 8 to 10 times that of an AMPS analog system
and 4 to 5 times that of a GSM system
2.Improved call quality, with better and more consistent sound as
compared to AMPS systems
3.Simplified system planning through the use of the same frequency in
every sector of every cell
4.Enhanced privacy
5.Improved coverage characteristics, allowing for the possibility of
fewer cell sites
6.Increased talk time for portables
7.Bandwidth on demand
=============================================================================
Nguyen Hoang Ha
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ASIAN INSTITUTE OF TECHNOLOGY
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